The present invention relates to nylon polymer useful for the manufacture of shaped articles such as fibers and injection molded articles and more particularly relates to nylon containing nucleation additives.
A common problem in the melt-spinning of highly crystalline nylon homopolymers such as poly(hexamethylene adipamide) has been the formation of spherulites in the polymer of the resulting fiber. Spherulites are spherical crystalline bodies of radiating crystals found in the polymer which typically form due to inorganic and degradation-related impurities. When large spherulites form in substantial numbers, they lower the strength or form defects in the fiber, either of which can cause the fiber to break in the spinning process or in end-use processing. Spherulites in the yarn are often responsible for defects or poor uniformity in the finished good made from the yarn which adversely affect the properties of the goods for their intended end use. Nylon carpet yarns containing pigments or other polymers such as polyolefins which are immiscible in the nylon are particularly prone to problems caused by spherulites.
Copolymer nylons crystallize less readily than homopolymer nylons and consequently are not as prone to the formation of spherulites. However, some of these yarns have a reduced levels of fiber finish pick-up which causes filament instability in the spinning process due to varying frictional forces experienced by the yarn on process equipment. This may result in a yarn with decreased uniformity which may in turn cause poor dye uniformity in fabric made from the yarn.
In the manufacture of molded articles by injection molding, one rate-controlling step in the process is the time which the mold is closed until the polyamide is sufficiently solidified to be removed from the mold without damage. It is desirable, therefore, to have a polyamide which crystallizes and solidifies quickly while, at the same time, preserves polymer clarity for applications where polymer clarity is desired.
Agents are known for addition to nylon polymer to affect the nucleation of the polyamide, i.e., the process by which crystallization takes place in the molten polymer. For example, as disclosed in U.S. Pat. Nos. 3,755,221, 3,529,929 and 4,176,227, particles of CaF.sub.2 are either added to or formed in the molten nylon polymer in situ to improve cycle times for injection molding or enable the use of higher spinning temperatures and higher extrusion rates in the melt-spinning of nylon fiber. However, such particulates are often ineffective and, when CaF.sub.2 is used, the fluoride ion in the reactants used to make CaF.sub.2 or fluoride ion in the molten polymer can cause corrosion of processing equipment. The particles of CaF.sub.2 also make the molded article or fiber have a cloudy appearance which is undesirable for some end uses.
Calcium acetate has also been added to a nylon 6,6/nylon 6 copolymer to reduce spherulites while maintaining clarity in the fiber as disclosed in U.S. Pat. No. 4,919,874. However, the presence of calcium acetate in the polymer substantially decreases the relative viscosity (RV) of the nylon polymer. Consequently, calcium acetate as a nucleating agent is generally undesirable and is entirely unsuitable for some processes for which high RV nylon polymer is required.